Temperature between Galactic Structures

So I’m trying to understand a bit more about the structure of the universe and I keep running across all these “if you think that’s strange wait until you hear this” moments and they almost always cause me an almost painful paradigm shift (without a clutch). Here is the latest.

I was always comfortable with the idea of the coldness of space. At about five molecules per liter of volume and so damn cold. Then I learn about the center of our galaxy and the super black hole lurking there and that the same type of thing is also lurking in most if not all of the other galaxies and that because of Hawking radiation that part of our neighborhood is blinding hot.

So not all of space is dead cold but certainly the space between galaxies is as cold as space should be in my mind at least and I was pleased to understand that – yes between galaxies is a cold place.

But what about the space between collections of galaxies like our own dear Local Group; the thirty of more galaxies like our own Milky Way and then next one over - the M81 group? Is it hot between them again? Filled with plasma of gamma rays or other such nasty surprises or is it cold as space?

Hawking radiation (unruh radiation) is not responsible for heating in the ISM/IGM. Yes, the space between galaxies is generally very hot, [tex]10^6[/tex]K degrees or so actually. We call that material the intergalactic medium (IGM). The interstellar medium (ISM) is also pretty hot in general. Well, the temperature varies quite a bit, from a few K in cold molecular clouds to [tex]10^4[/tex]K in H II regions. Generally its a few
[tex]10^3[/tex]K. The heating and cooling processes in the IGM/ISM are complex but its thought that supernovae are generally responsible for heating the gas in the IGM.

The space between galaxies is believed to have been heated by the universe's first stars, which were huge and millions of times brighter than our Sun.

It would still feel cold for a human out there because solid bodies radiate heat very rapidly. The density of gas is so low that it would not have a significant warming effect on you. You would quickly cool to near absolute zero even though the temperature of the gas is thousands of degrees.